1. Field of the Invention
The present invention relates to a device for controlling the concentration of the developer employed in recording apparatus such as a copier or a laser beam printer, thereby maintaining a constant optimum image density in such recording apparatus.
2. Description of the Prior Art
Developer composed of a mixture of toner particles and carrier particles is employed in certain recording apparatus such as dry-process electrophotographic copier and electrostatic recording apparatus. As an example of such recording apparatus, FIG. 1 schematically shows the structure of a conventionally known laser beam printer, in which a laser beam 17 generated by a laser device 15 is modulated by an A/0 modulator 16 in response to video signals 13 corresponding to the information for printing stored in a page memory 11. The laser beam 17 thus modulated is reflected by a polygonal mirror 25 rotated in a direction a for effecting principal scanning, then focused by an f-.theta. lens 27 and guided by a deflecting mirror 29 to a photosensitive member 33 essentially composed of a conductive substrate, a CdS photoconductive layer and an insulating laryer and provided on a photosensitive drum 31.
Said photosensitive member 33 of the drum 31 rotated in a direction b for auxiliary scanning is at first uniformly charged to a potential of several thousand volts by a corona discharge from a primary charger 41, then is exposed to the laser beam 17 modulated by the A/0 modulator 16 according to the video signals 13 simultaneously with the charge elimination with a charge eliminater 42 by an AC corona discharge or a DC corona discharge of a polarity opposite to that of the primary charging, and is uniformly illuminated by an exposure lamp 44 to form an electrostatic latent image of an elevated contrast on said photosensitive drum 31. The photosensitive member 33 bearing said electrostatic latent image shows locally varied electrostatic potential.
The electrostatic latent image formed on the photosensitive member 33 is developed into a visible image in a developing device 43 by depositing toner from developer 45 charged to a potential of several hundred volts. The toner image thus developed is transferred by a transfer charger 47 onto a transfer sheet 49 transported in a direction c, thus reproducing the information stored in the page memory 11 on said transfer sheet 49.
After the image transfer, a cleaning blade 51 removes the remaining toner and unrepresented means dissipates the remaining potential to prepare the photosensitive member for the succeeding image formation.
The developer 45 contained in the developing device 43 is generally a two-component developer composed of a mixture of toner particles and carrier particles, and the weight mixing ratio thereof has a significant influence on the image development to be achieved by the developing device 43.
For example, an excessively low toner concentration in the developer 45 will provide a low image density, while an excessively high toner concentration will give an excessively high image density combined with a background fog, eventually forming an unacceptable image on the transfer sheet 49.
Consequently, in order to constantly obtain an image of a desirable density, it is essential to maintain the toner concentration in the developer 45 at an appropriate and constant level during the image development.
There have been proposed various methods for maintaining a constant toner concentration.
One of such methods is to control the amount of toner replenishment in response to the optical detection of the image density after the development. Although this method allows control of the amount of toner replenishment in direct relation to the image density, it is inevitably associated with unstable density control because of the error in the detection caused for example by a smear on the optical sensor.
Other methods control the amount of toner replenishment through the detection of a change in the magnetic permeability or dielectric constant of the developer or through the detection of a change in the color of the developer composed of carrier and toner of different colors. Such methods are however unable to maintain the toner concentration at an appropriate level over a prolonged period, as they are apt to be influenced by the spent toner particles which are present in the developer but do not contribute to the developing process. Also such methods, not based on the direct measurement of the developed image density, are unable to compensate the image density by regulating the toner concentration in case the electrostatic potential on the photosensitive member shows a change after prolonged use to a level giving inappropriate image density.
Also there are known improved methods of detecting the change in the volume of developer 45 and effecting control to maintain said volume at a constant level, as disclosed in Japanese Laid-Open Pat. No. Sho 50-19459 entitled "Method for detecting and controlling the concentration of electrophotographic developer" and in Japanese Laid-Open Pat. No. Sho 51-78343 entitled "Electrophotographic developing device". Again referring to FIG. 1, these methods are based on the fact that the consumption of the developer 45 in the developing device 43 is mostly due to consumption of the toner and that the consumption of the carrier is zero or very small. Thus, assuming that the decrease in the volume of the developer 45 is caused solely by the consumption of toner or by the consumption of developer in which the consumption of toner is at a fixed level determined by the consumption of the carrier, these methods measure the decrease in the volume of the developer 45 and feed a replenishing developer in which the toner concentration is 100% or said fixed level to maintain the developer 45 at a constant volume in the developing device 43, thereby maintaining a constant toner concentration in the developer 45.
In practice, however, the ratio of the consumption of toner and carrier is not constant but is dependent on the area of recording. Consequently the replenishing developer of a fixed toner concentration causes a change in the toner concentration in the developer 45 after a prolonged period. In order to resolve such drawback, there has been proposed a method of optically detecting the image density after development and replenishing the carrier when the image density becomes excessively high due to the consumption of the carrier. In such method, however, the image density will become excessively low if the carrier is excessively replenished by error, and it will become necessary in such case to immediately regulate the toner concentration in the developer 45 by an additional replenishment of the toner. However, in case the toner replenishment is controlled by the detection of decrease in the developer volume as explained above, the toner replenishment may not be conducted in such state since the volume of the developer 45 is already increased by the replenishment of the carrier. Consequently, in such state it becomes necessary to continue to use the recording apparatus with the thus lowered image density for a considerable period, as the removal of carrier along from the developer is generally quite difficult.